The present invention pertains to the structure of a passive optical network system, in which a plurality of subscriber connection devices share an optical transmission line, as well as a communication method therefor.
In order to transmit and receive large-capacity signals and data via a communication network, the attainment of higher speeds and larger bandwidths of communication networks is being advanced as well in the access network connecting subscribers to a communication network, and the introduction of a Passive Optical Network (hereinafter referred to as “PON”) system specified in Recommendation G.984.1 to G.984.3 et cetera of the International Telecommunications Union (hereinafter referred to as “ITU-T”) is aimed for. A PON is a system connecting an Optical Line Termination (hereinafter referred to as “OLT”) connected with a host communication network; and Optical Network Units (hereinafter referred to as “ONU”) accommodating a plurality of subscriber terminals (PCs or telephones) connected with a passive optical network consisting of a trunk optical fiber and branch optical fibers. Specifically, it is a system carrying out communication with a mode in which the signals coming from the terminals (PCs and the like) connected to each ONU are sent as optical signals from branch optical fibers via an optical splitter and optically multiplexed (by time division) with the trunk optical fiber to the OLT and the OLT carries out communication processing of the signals from each ONU and either transmits them to the host communication network or transmits them to another ONU connected to the OLT.
The development and introduction of the PON started from a system handling signals with low speeds such as 64 kbit/s and is now proceeding with BPON (Broadband PON) transmitting and receiving fixed-length ATM cells at speeds of at most approximately 600 Mbit/s or EPON (Ethernet PON) transmitting and receiving variable-length Ethernet® packets at speeds of at most approximately 1 Gbit/s, and GPON (Gigabit PON) handling higher-speed signals on the order of 2.4 Gbit/s and standardized in ITU-T Recommendations G.984.1, G.984.2, and G.984.3. Moreover, in the future, there is demanded the implementation of high-speed PON system capable of handling signals from 10 Gbit/s to 40 Gbit/s. As means of implementing these high-speed PON systems, there are investigated multiplexing methods such as TDM (Time Division Multiplexing) multiplexing by time division, WDM (Wavelength Division Multiplexing) multiplexing by wavelength division, and CDM (Code Division Multiplexing) multiplexing by code division, a multitude of signals. Further, the present PON has adopted TDM, but e.g. GPON has a structure using different wavelengths for uplink signals (from ONU to OLT) and downlink signals (from OLT to ONU) and, as for the communication of the OLT with each ONU, allocating signal communication time with respect to each ONU. Also, there has been an evolution from structures processing existing fixed-length signals to structures that also handle variable-length signals (burst signals) which handle burst-shaped signals of diverse classes (audio, image, data, et cetera) more easily. For high-speed PONs from now on, there are also being investigated various multiplexing methods, but TDM is in the process of becoming the main direction of investigation.
In each of the aforementioned PON modes, the distance from the OLT to the ONU differs, since ONUs are installed in subscriber residences interspersed in various places. That is to say that, since the optical fiber lengths of the trunk optical fiber and the branch optical fiber combined from the OLT to each ONU (the transmission distance) have a random variation, the transmission delay between each ONU and the OLT has a random variation, so even if each ONU transmits a signal, there is a possibility that the optical signals output from each ONU collide and interfere with each on the trunk optical fiber. For this reason, using e.g. the technology of ranging such as e.g. specified in Chapter 10 of Recommendation G.984.3, the system operates in each PON so as to regulate, after carrying out a measurement of the distances between the OLT and the ONUs, the delay of the output signal of each ONU such that the signal outputs from each ONU do not collide.
Moreover, the OLT operates, using technology referred to as Dynamic Bandwidth Assignment (hereinafter referred to as “DBA”), so as to designate the transmission timing for each ONU so that the optical signal from each ONU does not collide or interfere on the trunk optical fiber when deciding on the bandwidth for the signal authorizing transmission for each ONU on the basis of transmission requests from each ONU, after taking into account the delay quantities measured with the aforementioned ranging. That is to say that the PON is configured such that the operation of the communication is performed under the condition that the timing of signals transmitted and received between the OLT and each ONU is managed inside the system.
In the transmission and reception of signals between the OLT and each ONU, according to e.g. the specification of Ch. 8.8.3 of Rec. G.984.2, there are added to the data (also referred to as the payload): a guard time for interference prevention and consisting of at most 12 bytes at the head of the signal from each ONU making it possible for the OLT to identify and process the signals multiplexed on the trunk optical fiber and coming from each ONU; a preamble utilized for the determination of the signal identification threshold value of the receiver inside the OLT and for clock extraction, a burst overhead byte called a delimiter which identifies pauses in the received signal; and a PON control signal (also referred to as overhead or a header). Further, since each piece of data is a piece of variable-length burst data, there is also added, at the head of each piece of data, a header called a GEM (GPON Encapsulation Method) header for processing variable-length data. Moreover, in the signal from the OLT destined for each ONU, there is a structure in which, at the head of signals transmitted from the OLT to each ONU, a frame synchronization pattern for identifying the head; a PLOAM field transmitting monitoring, maintenance, and control information; and overhead (also referred to as a header) called a grant field designating the signal transmission timing of each ONU are added to the data multiplexed by time division and destined for each ONU, to make it possible for each ONU to identify and processing signals from the OLT. Further, in the data destined for each ONU, there is added a GEM header for processing variable-length data, similarly to the signals from the ONUs. Using the grant field, the OLT designates the authorized timing for the uplink transmission (beginning (Start) and end (Stop) of transmission) of each ONU in byte units. This authorized timing for transmission is referred to as a grant. And then, when each ONU transmits data destined for the OLT during the concerned authorized timing, these are optically (by time division) multiplexed on the optical fibers and received by the OLT.